As a recording apparatus, which prints an image processed by a computer in multicolor and multi-tone (gradation), there is known an inkjet printer (hereinafter, referred to as an IJ printer) which records (forms) an image (also referred to as a dot image) on a recording paper as a recording medium by injecting ink droplets from nozzles provided to a recording head onto the recording paper. Means for generating energy to inject ink droplets in the IJ printer is generally classified into two methods, one being a so-called bubble jet® method (hereinafter, referred to as a BJ method) and the other being a so-called piezo method (hereinafter, referred to as a PZT type).
The BJ method propels droplets of ink by rapidly heating the ink filled in a pressurizing liquid chamber by utilizing a volume change by film boiling. The PZT method propels droplets of ink by a pressure due to a volume change generated by deforming a diaphragm constituting a part of a pressurizing liquid chamber by displacement of a piezoelectric element. Accordingly, although the pressurization of ink is different between the BJ method and the PZT method, there is no difference in the recording process to form a dot image by injecting ink droplets.
In the meantime, in order to output image data generated by a computer or acquired from an image input apparatus such as a digital camera or a scanner from the IJ printer, various processes must be applied to the image data. Generally, a process flow of image processing is such as shown in FIG. 1.
FIG. 1 is a flowchart of a process of converting input image data into data, which can output by a recording apparatus. In FIG. 1, CMM conversion processes P2 and P6 convert a color system (RGB) of image data P1, which is input as first image data, into a color system (CMY) of an output apparatus such as an IJ printer. BG/UCR processes P3 and P7 separate a black (K) component from the CMY data so as to generate second image data. γ-correction processes P3 and P7 adjust the output balance of each color component separated from the CMY data. Zooming processes P4 and P8 enlarge or reduce the image data so as to match the image data to an output resolution of the IJ printer. Halftone processes P4 and P8 compare each pixel in the enlarged image data with a threshold matrix so as to convert the image data into dot pattern data. An output process P10 outputs the dot image data as third image data.
Usually, image data sent from a personal computer PC has an amount of information with 8 bits for each color RGB per one pixel. Such an amount of information enables expression of 256 gradations from 0 to 255 for each color component.
However, all 256 gradations cannot be used in practice. Since output characteristic of each color component, cyan (C), magenta (M), yellow (Y) and black (K) is dependent on coloring materials used in ink, an injection characteristic of ink droplets and characteristics of recording medium (recording papers), ideal output characteristics cannot be always acquired. Although the γ-correction processes P3 and P7 are applied so as to correct the output characteristics, the γ-correction may cause a lack of gradations mentioned below.
FIGS. 2A through 2F are graphs for explaining a lack of gradations due to a gamma-correction and avoidance of such a lack by an expansion of a gradation level. In FIGS. 2A through 2F, a horizontal axis represents an input tone (input gradation) and a vertical axis represents an output tone (output gradation). FIG. 2D is an enlarged view of a part indicated by a circle in FIG. 2C. FIG. 2F is an enlarged view of a part indicated by a circle in FIG. 2E. FIG. 2A shows a state where no correction is applied and an input and an output correspond to each other on one-to-one basis. On the other hand, since a nonlinear correction as shown in FIG. 2B is usually applied, a lack of gradations as shown in FIG. 2D, which is a partial enlarged view of FIG. 2C, may occur after the correction.
In order to solve such a problem of lacking in gradations, Japanese Laid-Open Patent Applications No. 2001-45308 and No. 2001-285630 suggest a process being performed after enlarging the data input to the CMM process into gradation data of more than 8 bits. Moreover, Japanese Laid-Open Paten Applications No. 2000-108384 and No. 2001-144958 suggest an expansion of a number of gradations with respect to data after the CMM correction. This treatment is to reflect a part, which is eliminated as a value less than a decimal in 8-bit data, as a gradation level.
FIGS. 2C and 2D show the lack of gradations, and FIG. 2E and 2F show an aspect in which the part of lacking gradations (shown in FIGS. 2C and 2D) is not eliminated and is reflected in the corrected data by the expansion of a number of gradations.
Thus, it is possible to avoid an occurrence of lacking gradations due to a gamma-correction by performing an expansion of a gradation level as mentioned above. However, it is natural that an expansion of a gradation level causes an increase in an amount of information. If an amount of information to handle is increased, a capacity of a buffer for operation must be increased. There may be a case where a expansion of a gradation level causes not only a decrease in a processing speed due to an increase in an amount of operation but also a cost increase due to an increase in a buffer capacity.
In recent IJ printers, a number of nozzles mounted on one head unit tends to be increased so as to acquire both a high-resolution and a high-speed operation. Especially, in a BJ printer that is capable of being applied with a photolithography technique, a head having more than 20 nozzles has become popular. Although such an increase in a number of nozzles is to avoid, by increasing the number of nozzles, a problem in that a recording width per a unit number of nozzles is decreased due to a decrease in a nozzle pitch to acquire a high-resolution, an amount of information to be sent to a head at once is increased by an amount corresponding the a number of nozzles increased.
Further, since it is necessary to increase a resolution not only in a sub-scanning direction, which is a direction of adding nozzles, but also in a main scanning direction, an amount of data required for one scan of a head is increased as a square times of the number of nozzles. In association with an increase in an amount of operation in an image processing apparatus due to an increase in resolution, a time lag may be generated between a scan timing of a head and a time of completion of transfer of data to a head unit. In such a case, the operation of the head unit is stopped until necessary data is received, which results in deterioration in effects of the increase in the number of nozzles.